1. Technical Field
The present invention relates to elements for solid polymer electrolyte fuel cells, and relates to separators for forming gas passages in the fuel cells.
2. Related Art
In solid polymer electrolyte fuel cells, a separator is layered on both sides of a plate-shaped electrode to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The electrode is a three-layered structure, in which a polymerized electrolytic membrane made from a resin such as ion-exchange resin is held by a pair of gas diffusion electrode plates (positive electrode plate and negative electrode plate). The separator is formed with gas passages for flowing a gas between the gas diffusion electrode plate. According to the fuel cell, hydrogen gas as a fuel is provided to the gas passages facing the gas diffusion electrode plate at the negative electrode side, and an oxidizing gas such as oxygen or air is provided to the gas passages facing the gas diffusion electrode plate at the positive electrode side, whereby electricity is generated by electrochemical reaction.
Conventional materials for separators can be classified approximately into the carbon type and the metal type. As materials of the carbon type, gas impermeable carbon in which a resin such as phenol is impregnated into a sintered isotropic carbon, an amorphous carbon in which a resin such as phenol is formed and baked thereafter, a composite formed material of a resin and carbon, and the like, are mentioned. As material of the metal type, high corrosion resistance metals such as stainless steels, titanium alloys, materials in which these metals are coated by noble metals such as gold or platinum, and the like, are mentioned. The above materials of the carbon type have superior characteristics, high corrosion resistance and excellent stability in environments of fuel cells, good electrical conductivity, and low electrical resistance. In contrast, the materials of the metal type have superior mechanical strength.
However, the materials of the carbon type and metal type have the following problems.
The gas impermeable carbon in materials of the carbon type is hard, and the gas passages are not easily formed by machining, thereby requiring much labor and expense. In the amorphous carbon, the sizes of products are not uniform since it contracts and deforms during baking. In a composite material of a resin and carbon, mechanical strength and impact strength are low, whereby cracks often occur in forming and assembling. In contrast, in the materials of metal type, since gas passages are formed by press forming, or the like, complicated and fine gas passages for obtaining the maximum generation efficiency cannot be easily formed.
Therefore, an object of the present invention is to provide a separator in fuel cells, which can satisfy the characteristics of the materials of the carbon type such as corrosion resistance and electrical conductivity and mechanical strength equipped by the materials of metal with high performance, and in which gas passages can be easily formed and manufacturing efficiency can be improved.
The present invention provides a separator for a fuel cell comprising a plate-shaped electrode having a pair of gas diffusion electrode plates and an electrolytic layer held by the gas diffusion electrode plates; the separator being layered on both surfaces of the electrode and forming gas passages cooperating with the gas diffusion electrode plate. The separator comprises: a metallic plate; protrusions made from a material of the carbon type, the protrusion having a projected surface projecting from the metallic plate toward the gas diffusion electrode plate so as to contact therewith and to form the gas passages; and intermetallic compounds precipitated on the projected surface of the protrusion. The gas passage is formed by a surface of the metallic plate and a pair of side surfaces of adjoining protrusions.
According to the separator of the invention, the metallic plate is a substrate, and in particular, the thin plate portion of the gas passage is formed by the metallic plate. Since stress is concentrated at the thin plate portion of the gas passage, the mechanical strength can be improved, and the structure can therefore be thin, light weight, and compact. Since the separator is impermeable to gas due to the metallic plate, reduction of the generating performance due to gas leakage can be prevented. The protrusion contacting with the gas diffusion electrode plate is formed from a material of the carbon type, so that the electrical resistance is small, and the electrical conductivity can be improved. Since the intermetallic compounds project from the metallic plate, the corrosion resistance thereof is improved. Furthermore, the protrusion is made from a material of the carbon type carbon, and the overall corrosion resistance can be improved.
As the metallic plate in the invention, for example, thin plates made from stainless steels, titanium alloys, or the like, are preferable. As materials of the carbon type, for example, expanded carbon, composite materials of the expanded carbon and a resin, composite materials of carbon and a resin, sintered carbon, amorphous carbon, composite materials of a carbon fiber, composite materials of a carbon fiber and carbon, or the like, are preferable.
The materials of the carbon type will be explained hereinafter. The expanded carbon is provided with flexibility by refining natural carbon as a raw material and subjecting it to an acid treatment, and then expanding it in the direction of the C axis (direction along the distance between the hexagonal faces of the carbon crystal) under high pressure and high temperature. The expanded carbon may be formed into the required shape by press forming, or the like. A composite material of the expanded carbon and a resin may be produced by mixing a binder resin such as phenol with the expanded carbon during press forming the expanded carbon. A composite material of carbon and a resin may be produced by mixing a powder of artificial carbon and a binder resin such as phenol and compressing and forming the mixture. The sintered carbon may be produced by mixing and press forming coke as a filer and pitch as a binder, and sintering it at a temperature in the range of 2000 to 2500xc2x0 C., and then densifying it by repeatedly impregnating the pitch and sintering it. Amorphous carbon may be produced by forming phenol resin and baking it at a temperature of around 1500xc2x0 C. The composite material of carbon fiber may be produced by compressing and forming a carbon material of a fiber with a carbon content of 90% by weight or more together with a binder such as phenol resin. The composite material of carbon fiber and carbon may be produced by sintering the composite material of carbon fiber at a temperature of 2000xc2x0 C. or more.
The protrusion made from a carbon material and projecting from the metallic plate is secured to the metallic plate by various means. For example, the protrusion may be pre-formed and secured to the metallic plate by means of adhering or press adhering. Alternatively, a material powder of carbon type may be compressed or injected into the metallic plate so as to compact the protrusion, and it is then sintered so as to secure it to the metallic plate. In this manner, when a through hole for engaging the protrusion therewith is formed in the metallic plate, the securing strength of the protrusion with respect to the metallic plate is improved, and the electrical resistance is decreased and the electrical conductivity is improved. According to the forming method and the securing method for the protrusion, complicated and fine gas passages can be easily formed, and generating performance can be improved.
The protrusion secured to the metallic plate by the above manner contacts with the intermetallic compounds precipitated on the surface of the metallic plate. Since the intermetallic compounds have a low electrical resistance, the contact resistance is extremely low in comparison with the case in which the protrusion contacts with the matrix of the metallic plate. Therefore, the fuel cell demonstrates good electrical conductivity and high generation performance. As intermetallic compounds with low electrical resistance, carbides of the Fexe2x80x94Cr type may be mentioned. Therefore, it would be very effective if the metallic plate is made from stainless steels and carbides of the Fexe2x80x94Cr type are precipitated on the surface to which the protrusion is secured.